Gas bottle lockers are notorious for the flimsiness of their flooring - often not much more than some pressed cardboard. OK when it is dry and there is something stronger beneath, (for example where the A frme runs) but not when wet. Do check the flooring in the van itself, but it is unlikely that the feeble flooring is fitted anywhere but that locker. I would try reinforcing to bridge across the A frame, then lightish waterproof ply as a floor - you may be able to use the old flooring as a template. Difficult, fiddly place to work in, though.